1. Field of the Invention
The present invention relates to an ink jet printing head, and particularly to a printing head that ejects ink with a thermo-mechanical actuator.
2. Description of the Related Art
Conventionally, as an ejection method of printing heads used in ink jet printers and the like, for example, a thermal jet type which ejects droplets of printing liquid through a bubble caused in the printing liquid by means of an electric resistance heater and a piezoelectric type which mechanically pressurizes the printing liquid, are in practical use. On the other hand, a thermo-mechanical actuator type, with its two advantages of being of low cost due to the use semiconductor manufacturing as the thermal jet type and the high degree of freedom for available printing liquid as used in the piezoelectric type, is in use as a known method for ejecting ink (refer to Japanese Patent Laid-Open No. 2004-160650).
A thermo-mechanical actuator has a cantilever element formed of a plurality of layers having different thermal expansion coefficients. The current flowing into each layer of the cantilever elements is controlled so that the lever element bends due to the different thermal expansions for generating the ink ejection pressure.
For the cantilever elements, various types have been proposed, for example, a cantilever element with a tapered shape is known and intended to improve ejection energy efficiency (refer to Japanese Patent Laid-Open No. 2004-082733).
FIGS. 6A to 6D are views showing ejection part structures and an ejection operation of printing heads using conventional thermo-mechanical actuators. FIG. 6A is a side cross-sectional view of a common liquid chamber H160, which communicates with each of the plurality of ejection portions arranged in a printing head to supply ink to them, and one ejection portion H111. FIG. 6B is a view of the ejection portion viewed from an ejection opening side with the members forming the ejection opening being removed.
As shown in these figures, the ejection portion H111 has two chambers, a first separate liquid chamber H220 and second separate liquid chamber H221, which communicate with each other, and each of which is connected to the common liquid chamber H160. A cantilever element H230 is provided in a cantilevered state in which one end of the element is fixed to the inner wall surface of the first separate liquid chamber H220 and at the same time the element extends to the inside of the first separate liquid chamber H220 and the second separate liquid chamber H221. As shown in FIG. 6B, when viewed from the common liquid chamber, the first separate liquid chamber H220 has a rectangular cross-section and the second separate liquid chamber H221 a circular cross-section. The second separate liquid chamber H221 is provided with an ejection opening H240 (shown by a dashed line in FIG. 6B) on the wall surface that forms the chamber H221 and is opposite to the common liquid chamber H160 relative to the cantilever element H230.
The cantilever element H230 has a three layered structure of a first deflector layer H231 and a second deflector layer H232 with relatively large thermal expansion coefficients, and a barrier layer H233, which is interposed between the first deflector layer H231 and the second deflector layer H232 and is made of a material with relatively low thermal conduction rate and smaller thermal expansion coefficient.
As shown in FIG. 6B, the planar shape of the cantilever element H230 has a region of trapezoid which is located inside the first separate liquid chamber H220 and expands toward the fixed side at the inner wall surface of the first separate liquid chamber H220. Also a region located inside the second separate liquid chamber H221 has circular shape. The first deflector layer H231 and the second deflector layer H232 are electrically connected to a wiring portion H250 at the outside of the first separate liquid chamber H220. Thereby, when ejecting ink, the electric pulses required for ejecting ink are applied to the first deflector layer H231 and the second deflector layer H232.
As above mentioned, the cantilever element H230 has a triple layered structure comprising the first deflector layer H231 and the second deflector layer H232 with large thermal expansion coefficients, and the third deflector layer H233 interposed between them, which is made of a material with low thermal conduction rate and small thermal expansion. Therefore, when electrical pulses are applied to the first deflector layer H231 or the second deflector layer H232, the first deflector layer H231 or the second deflector layer H232 is heated and expands according to the heat. In this case, the first deflector layer H231 or the second layer H232 bends toward the barrier layer H233 because the first deflector layer H231 or the second layer H232 is closely contacted to the barrier layer H233, which does not expand very much by the heat. This bending motion pressurizes the ink inside the second separate liquid chamber H221 and thus ejects the ink.
More specifically, in a usual state (non-ejecting state), the cantilever element H230 remains horizontal as shown in FIG. 6A.
By applying predetermined electrical pulses to the second deflector layer H232 in the usual state, the second deflector layer H232 is heated and then expands by that heat. And at this time, as the barrier layer H233 does not expand very much, it functions in restricting the expansion of the second deflector layer. As a result, the cantilever element H230 bends in the direction away from an ejection opening H240 (state in FIG. 6C). Through this action, the ink around the ejection opening H240 moves toward the upper part in FIG. 6C and forms a meniscus around the ejection opening H240. This enables the preparation of a good ink drop formation for the next ejecting action.
Thereafter, the second deflector layer H232 is gradually cooled down. At this time, by applying predetermined electrical pulses to the first deflector layer H231, the cantilever element bends similarly to the above but toward the ejection opening H240, which direction is opposite to the direction in the above described action. And through this action, the ink in the second separate liquid chamber H221 is pressurized and ejected as an ink droplet from the ejection opening H240.
Moreover, after that, as the first deflector layer H231 is cooled down, the cantilever element H230 returns to its regular horizontal state (state in FIG. 6A). By repeating all the above actions, the continuous ejection can be performed.
Moreover, reference signs H261, H262, H263 and H264 designate connection portions of respective wirings H251, H252, H253 and H254 for applying the electrical pulses to the first and second deflector layers. For example, a set of the wiring H251 and the connector H261 and a set of the wiring H252 and the connector H262 connect to the first deflector layer. Also, a set of the wiring H253 and the connector H263, and a set of the wiring H254 and the connector H264 connect to the second deflector layer. This connection arrangement allows the desired electrical pulses to be applied to respective deflector layers.
In the ejection action of the thermo-mechanical actuator described above, the amount of ink in the second separate liquid chamber H221 is decreased. This reduced amount of ink is then refilled from the first separate liquid chamber H220 rather than directly from the common liquid chamber H160 which is located above the second separate liquid chamber. The reason comes from that there is the large flow resistance when ink flows across the cantilever element vertically in the second separate liquid chamber H221, because the cross sectional shape of the second separate liquid chamber H221 and the planar shape of the cantilever element H230 are identical and the gap between them is very small.
When thus refilling the ink from the first separate liquid chamber H220, the ink is refilled from where it is most easily to flow in, that is, from the nearest place to the second separate liquid chamber H221 of the structural positions of the first separate liquid chamber H220. The flow of ink, as the arrows A3 and A4 shown in FIG. 6B, goes across the cantilever element H230 from the upper part of the first separate liquid chamber H220 and goes into an under part of the second separate liquid chamber.
However, in the above conventional example, since in the separate first and second liquid chambers the flow of ink caused during every ejection is shown as arrows A3 and A4 in FIG. 6B, the ink in a part of the first separate liquid chamber H220 which part is far away from the second separate liquid chamber H221 does not flow so much and stagnates. Also, this part is located far from where the bending action of the cantilever element directly causes the ink to flow. This also causes the ink in this part to stagnate easily. Therefore, in this part, bubbles generated by ejection operation, etc., easily accumulate. If the accumulated bubbles exceed a given amount, the bubbles then reach the second separate liquid chamber H221 and cause the ejection failure due to the bubbles being taken into the ejection opening H240.
Usually, residual bubbles in the liquid chamber are removed through the recovery action caused by the suction of ink through the ejection opening. However, with liquid chamber structures in which bubbles accumulate and likely to stagnate, removal of the bubbles is difficult even with the recovery action and then the ejection failure sometimes may occur.